The Curiosity rover, is, by general consensus, pretty cool. The peripatetic planetary explorer, aka the Mars Science Laboratory, not only experienced one of the most dramatic and famous landings of all time when it arrived on the Red Planet last summer, it also showed the world was probably habitable in its past. It’s only natural, then, that while Curiosity giveth, sometimes it can also taketh away — in this case, interesting amounts of methane serving as possible evidence of sub-surface bacteria.

Mars Meth(ane) Heads

Perhaps most familiar as a source of energy and heat, methane (CH4) is also common throughout the solar system. Some planets have naturally occurring methane, but most of Earth’s supply of the gas is a result of biological processes. No wonder it’s interesting to Mars researchers: a high amount of the gas could indicate life on the Red Planet to produce it, in the form of underground microbial activity.

Scientists’ extensive observations of Mars, both from Earth and from instruments in Mars orbit, had shown methane to exist there in varying amounts. But not varying over time — varying from observation to observation. From highs of 45 parts per billion by volume (ppbv) ten years ago to more recent observations (over the past two years) of just 7-8 ppbv, there was no clear consensus on methane concentrations in Mars’ atmosphere. In fact, at Gale Crater, Curiosity’s landing site, one set of instruments recorded approximate methane amounts of 15 ppbv in fall to 30 ppbv in winter, while another found 30 ppbv in fall and 5 ppbv in winter.

Enter Sand-Robot

It’s a lucky thing, then, that Curiosity has an instrument that can specifically measure methane amounts (the Tunable Laser Spectrometer, part of the Sample Analysis at Mars suite). Surely, with its history of providing tantalizing information revealing the Red Planet’s life-harboring abilities, the roving robot’s methane analysis would be similarly upbeat, right?

Nope. “To date we have no detection of methane,” write the authors of a study in today’s Science, announcing Curiosity’s disappointing findings. Their figures indicate a maximum amount of just 1.3 ppbv for methane in Mars’ atmosphere. It’s surprising but not shocking, since there was already so much confusion about the methane levels. (Plus a fair amount of controversy, since anything that has to do with extraterrestrial life often leads to emotional rather than rational discussion.) It’s possible the previous findings were accurate, and that some unknown process has destroyed or removed the methane (such as a dust devil creating an electric field that zaps away the gas), but there’s no evidence to support this.

So it looks like reports of underground life on Mars have been greatly exaggerated. To be sure, the odds of Mars currently hosting life were never enormous, but now they’re that much lower. (But still not zero!)

Curiosity’s findings don’t always prove breathtaking, but this one at least is still scientifically interesting. Why did all those previous observations measure so much methane? And now that methane isn’t likely a factor, what else can the rover look for that might suggest biological activity? Just because the answer’s “no” doesn’t mean the question wasn’t worth asking.